Method of demolishing under-water obstacles



Feb. 26, 1963 E. F. PONCELET 3,078,798

METHOD OF DEMOLISHING UNDERWATER OBSTACLES Filed June 2,. 1960 BUBBLE DIAMETER IN FEET 2 4 6 8 IO 20 4O 6O QO IOO INVEN TOR. E. F. PONCELET DEPTH OF WATER IN FEET ATTYS.

United States Patent fiice 3,078,798 Patented Feb. 26, 1963 3,078,793 METHOD OF DEMOLlSHING UNDER- WATER GBSTACLES Eugene F. Poncelet, Palo Alto, Calif., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed June 2, 1960, Ser. No. 33,603 7 Claims. (Cl. 102-23) This application relates to the building and construction art and is more particularly concerned with a novel mode of removing underwater obstacles with a minimum of explosive material.

It was believed by those skilled in the art that to destroy an underwaterobst-acle, it was necessary to use a quantity of high explosive greater than that employed in destroying the same block in air. A charge just sufficient to destroy an obstacle in air generally fail to perform its function under water.

Accordingly, it is an object of this invention to provide a new method for the demolition of underwater obstacles which uses substantially the same amount of explosives as required to demolish the obstacle in air.

This object and many other objects will become more readily apparent to those skilled in the art after reading the following specification and considering the appended drawings wherein like numerals designate like or similar parts throughout the various views, and in which:

FIG. 1 is a diagrammatic representation of an explosive system constructed according to the principles of this invention in position about an obstacle to be destroyed;

FIGS. 26 indicate the mode of propagation of the destructive forces throughout the obstacle and;

FIG. 7 is a graph which indicates the relation between the variables governing the amount of explosive employed in practicing this invention.

During explosive breakup of an underwater obstacle most of the kinetic energy of the particles impelled by pressure waves induced upon the detonation of a demolition charge is expended in propagating the pressure wave in water. This leaves little or no energy to fracture the obstacle. Accordingly, in order to reliably and satisfactorily demolish the obstacle which could be demolished by a given size explosive charge in air, it was formerly necessary to increase the weight of the charge in order to insure complete breakup of the obstacle under water.

A principal feature of this invention resides in the fact that it is necessary to surround an underwater obstacle with a low density medium, such as a gas bubble in order to use a minimum amount of explosive. This reproduces the conditions under which the obstacle is fractured while surrounded by air.

FIG. 1 shows a typical obstacle such as a concrete block 11 viewed from the top and surrounded by water 21. A main charge 13 is disposed at one surface of block 11 While a plurality of small auxiliary charges 12 are disposed at the remaining exposed surfaces of block 11. Auxiliary charges 12 are connected to the main charge 13 by Primacord delays shown at 22 to allow the bubbles formed by the small charges to reach their optimum expansion prior to the initiation of the main explosive charge.

Upon ignition of charges 12 by any suitable means, not shown, a series of gas bubbles 14 are formed and begin to expand prior to the initiation of main charge 13'. This is shown in FIG. 2. After a delay of several milliseconds, the main charge 13 is detonated and the expanding gas bubbles 14 coalesce with the gas bubble generator by the detonation of the main charge to form a gas envelope 17 which completely surrounds the block. This can be seen in FIGS. 3 and 4. While the block 11 is com- Ind pletely surrounded by the gas bubble, the expanding pressure pulse 16 generated by the explosion of the main charge drives across the block and a strong rarefaction pulse 18 develops behind pulse 16 as the pressure pulse is reflected from the surface of the block.

It is essential that, as pressure pulse 16 moves across block 11 to the outer surfaces of the block, the water normally contiguous to these surfaces be removed. As can be seen in FIG. 3, the air bubble generated at the upper and lower surfaces of block 11, reaches the corners 10 no later than the arrival of pressure pulse 16 at these corners. Accordingly, the pulse 16 is reflected as strong rarefaction 18 from the upper and lower surfaces of the block 11. If the block were surrounded by water in substantial contact therewith the water would exert a tamping effect and the energy of pulse 16 would be propagated through the water rather than being reflected internally of the block. Accordingly, by surrounding block 11 with an air bubble, the destructive charge is utilized in the optimum manner.

Pressure pulse 16 is reflected from the far surface and the rarefaction pulses 18 present the situs for the development of a primary crack 19 in the block 11 which begins to form at the intersection of the pulses reflected into the block. Immediately following the formation of the primary crack the entire block disintegrates completely inside gas bubble 17.

By this invention the energy of the explosive is concentrated in disintegrating the obstacle rather than propagating the pressure wave in the water.

It should be obvious that instead of using the small charges 12, the obstacle 11 may be surrounded with a series of turns of Primacord. The use of Primacord, however, is not the most economical mode of performing this invention as it takes an appreciable time for the bubble to cover the whole surface of the concrete unless a large quantity of the Primacord is used.

The weight of the main charge is governed by the size of the obstacle; generally, it may be about 20 pounds or multiples thereof. The auxiliary charges may weigh from a small fraction of a pound to several pounds. The precise weight of the auxiliary charges is governed by several factors: the depth of water at which the obstacle rests, the size of the underwater obstacle (which determines the maximum bubble diameter required), and the half period of bubble oscillation (which determines the delay necessary between the initiation of the auxiliary charges and the initiation of the main explosive charge).

The interrelation of these factors is shown in the graph of FIG. 7 wherein the ordinate denotes the maximum bubble diameter in feet and the abscissa denotes the depth of water in feet. Line A represents a weight of each auxiliary charge of one-quarter pound while lines B and C represent auxiliary charges of one half and one pound, respectively. For comparison, lines E, F and G are shown on the graph to indicate delays of 100, and 50 milliseconds, respectively. Of course, the delay required may be some other value than that illustrated. The relation between W, T, H and D is governed by the equations:

where W represents the charge weight in pounds, T represents the half period of bubble oscillation in seconds (delay), H represents the depth of water in feet and D represents the maximum bubble diameter in feet.

Of course, if the depth of water is very great the weight of the auxiliary charges must increase; similarly if the size of the obstacle is great the maximum bubble diameter must also be sufliciently great to enclose it. Accordingly,

the weight of the auxiliary charge used must be correspondingly large. Generally speaking, the delay should be of the order of twenty-five to one-hundred milliseconds. Knowing this and the depth of the water and the size of the object, the weight of the charge may be readily determined by reference to FIG. 7. For example, if the depth of water is twenty feet and the delay time is 50 milliseconds and the size of the object is such that a bubble of slightly more than four feet in diameter will enclose it, the weight of the auxiliary charges may be. only one-quarter pound. a

The time delay can be adjusted by varying the length of delay Primacord that connects the auxiliary charges to the main explosive charge. it is essential that when the shock wave arrives at a surface of the block, that surface be surrounded by a gas bubble. This prevents damping of the shock wave and promotes internal refiection of its energy in the form of a raret'action pulse. Therefore, the time sequence of detonation of the auxiliary charges with respect to the main charge and with respect to each ether is not critical so long as the surface of the obstacle normally contiguous to the water is' surrounded by a gaseous medium when the pressure pulse from the main charge arrives at that surface and, therefore, the main charge may at times be detonated before the auxiliary charges depending upon the configuration of the obstacle and the physical relationship of the auxiliary charges to the main charge. The auxiliary charges are placed in such manner that the bubble 14 generated by the detonaticn of these charges coalesces to form the large bubble 17 at about the time shock wave 16 reaches the right surface of block 11 as seen in the drawing.

It should be apparent to those skilled in the art from the foregoing description of this invention that it is susceptible of many modifications and alterations without departing from the spirit and scope thereof. Accordingly, the invention is not to be construed as limited in any manner by the foregoing illustrative examples but is to be defined only by the scope of the appended claims.

What is new and desired to be secured by Letters Patent of the United States is:

1. A method of destroying an underwater obstacle which comprises the steps of progressively surrounding the obstacle with a gas bubble, and detonating a destructive charge at an outer surface of the obstacle opposite the bubble while the obstacle is being surrounded by the gas bubble.

2. The process of destroying an underwater obstacle which comprises the steps of substantially surrounding the obstacle with a plurality of auxiliary explosive charges, disposing a main explosive charge against an outer surface of the obstacle intermediate a pair of adjacent auxiliary charges, detonating the auxiliary charges to form a' plurality of expanding gas bubbles about the obstacle, detonating the main explosive charge subsequent to the detonation of the auxiliary charges and while the bubbles are about said obstacle, thereby to obtain optimum utilization of said main explosive charge.

3. The process of destroying a concrete block completely surrounded by water, comprising the steps of posis tioning a plurality of auxiliary charges adjacent to said block and in abutting relation therewith, disposing a main explosive charge at an outer surface of said block intermediate a pair of adjacent auxiliary charges, detonating the auxiliary charges to form an expanding gas bubble, detonating the main explosive charge at a time up to onehundred milliseconds later than the detonation of said auxiliary charges thereby to form a gas bubble completely surrounding said block, the detonation of said main explosive charge breaking up said concrete block while it is completely surrounded by said gas bubble.

4. The process of destroying an underwater obstacle which comprises; detonating an exposive charge at a surface of the obstacle to produce a shock wave internally traversing the obstacle, progressively surrounding the surface of said obstacle normally contiguous to the water with a gaseous medium in such a manner that as the shock wave arrives at a portion of said surface of the obstacle normally contiguous to water, said portion is surrounded by said gaseous medium.

5. The process of destnoying an underwater obstacle which comprises the steps of placing an auxiliary charge adjacent an outer surface of said obstacle, placing a main charge external to and adjacent said obstacle in such a manner as to generate a pressure pulse internally travers ing said obstacle upon explosion of said main charge, exploding said auxiliary charge and said main charge in such time sequence that when said pressure wave arrives at a portion of said obstacle normally contiguous with the water, said portion is surrounded by a gaseous medium formed by the explosion of said auxiliary charge.

6. The process of destroying an underwater obstacle comprising the steps of placing a plurality of auxiliary explosive charges adjacent a like plurality of outer surfaces of said obstacle so as to form upon explosion a. gaseous medium separating said outer surfaces from contact with the water, disposing a main explosive charge on an additional outer surface of the obstacle in a position with respect thereto so as to form a pressure pulse internally traversing said obstacle, exploding said auxiliary charges and said main charge in such time sequence that when said pressure pulse arrives at any portion of the outer surface of said obstacle normally contiguous with the water, said portion is separated from contact with the water by a gaseous medium formed by the explosion of said auxiliary charges.

7. The process of destroying an underwater obstacle which comprises the steps of partially surrounding the underwater obstacle with an expanding gas bubble contiguous therewith, and detonating an explosive charge at the outer surface of the obstacle outside the bubble to cause the bubble to coalesce with the bubble formed by the explosion of the charge and completely surround the obstacle, the force of the explosion being sulficient to break up and destroy the underwater obstacle while the obstacle is completely surrounded by the coalesced bubble.

References Cited in the file of this patent UNITED STATES PATENTS 1,248,689 MCAVOY Dec. 4, 1917 2,029,478 Haines Feb, 4, 1936 2,446,640 Davis Aug. 1O, 1948 2,586,706 Parr Feb. 19, 1952 

1. A METHOD OF DESTROYING AN UNDERWATER OBSTACLE WHICH COMPRISES THE STEPS OF PROGRESSIVELY SURROUNDING THE OBSTACLE WITH A GAS BUBBLE, AND DETONATING A DESTRUCTIVE CHARGE AT AN OUTER SURFACE OF THE OBSTACLE OPPOSITE THE BUBBLE WHILE THE OBSTACLE IS BEING SURROUNDED BY THE GAS BUBBLE. 